/Engine/Source/Runtime/UMG/Public/Blueprint/UserWidgetPool.h
In the case a pool has no owner widget, a default player controller can optionally be specified allowing greater control in split screen scenarios. The following priority rules are applied when determining the controller which should own created widgets: Controller of owning widget (if set) > Default player controller (if set) > First local player controller from world.
void SetDefaultPlayerController ( [APlayerController](API\Runtime\Engine\GameFramework\APlayerController) * InDefaultPlayerController )